Graphene is an individual sheet of graphite (“monolayer”) consisting of carbon atoms in sp2 hybridization mode with an arrangement according to a two-dimensional hexagonal lattice. It is one of the hardest materials known at present. At the interior of a graphene sheet, the hexagonal structure is predominant; however, some isolated units with a pentagonal or heptagonal structure may also be found, which constitute defects of the material, causing deviations with respect to a planar structure consisting exclusively of hexagonal patterns. Graphene structures formed by a plurality of superimposed sheets are also known: these structures have been called FLG (“few-layer graphene”). The thickest structures have been called NGP (“nano-sized graphene plates”). The distinction between these two terms is not very clear. The patent application WO 2005/084172 proposes the term “Carbon nanoflake” (CNF) for planar carbon forms with a thickness not exceeding 10 nm, and the term “Carbon nanosheet” (CNS) for CNFs with a thickness not exceeding 2 nm.
Most methods for obtaining graphene currently reported in the literature may be classified into five categories:
(A) The Obtaining of Graphene by a “Micromechanical” Exfoliation from HOPG (Highly Oriented Pyrographite).
This first method is described in the following articles: [1] K. S. Novoselov, A. K. Geim, S. V. Morozov et al.; Science (2004) 306, 666. [2] K. S. Novoselov, D. Jiang et al.; Proc. Natl. Acad. Sci. USA (2005), 102, 10451. [3] Y. Zhang, Y. Tan, H. L. Stormer, P. Kim; Nature (2005), 438, 201. [4] K. S. Novoselov, A. K. Geim et al.; Nature (2005), 438, 197. [5] K. I. Bolotin, K. J. Sikes, Z. Ziang et al.; Sol. State Comm. (2008), 146, 351. [6] F. Miao, S. Wijeratne, Y. Zhang, U. C. Coskon et al.; Science (2007), 317, 1530.
(B) The Obtaining of Graphene by Heating SiC at a High Temperature.
This second method is described in the following articles: [7] C. Berger, Z. M. Song, X. B. Li et al.; Science (2006), 312, (5777), 1191. [8] C. Berger, Z. M. Song, T. B. Li et al.; J. Phys. Chem. B (2004), 108 (52), 19912.
(C) The Synthesis of Graphene by Vacuum Deposition (CVD, Chemical Vapor Deposition) on a Metal Substrate.
This third method is described in the following articles: [9] J. Coraux, A. T. N'Diaye, C. Busse, T. Michely; Nanoletters (2008), 8, 565. [10] J. Vaari, J. Lahtinen, P. Hautojärvi; Catal. Lett. 55, 43 (1997). [11] D. E. Starr, E. M. Pazhetnov et al.; Surface Science 600, 2688 (1006). [12] M. Dresselhaus et al., Nano Letters, article in press (2009).
It is a synthesis in the traditional sense of the word, which proceeds chemically from gaseous compounds with a molecular mass lower than that of the product. As an example, according to a recent method described in reference [12], a hydrocarbon compound is decomposed on the surface of a metal (typically a substrate coated with a nickel film) to form a solid metal-carbon solution; after heating at high temperature, the carbon atoms segregate on the surface of the metal to form graphene sheets. The graphene sheets obtained by this method have relatively large sizes, on the order of several micrometers. However, the industrial production of graphene from this method appears to be difficult to implement.
A method for catalytic vapor deposition described in U.S. Patent Application Publication No. 2005/0255034 A1 (Wang and Baker) leads to nanofibers comprised of thin graphite plates superimposed in a direction perpendicular to the graphite planes.
(D) Synthesis of Graphene from Intercalated Graphite.
A fourth category of method uses, as the precursor, intercalated graphite of which the planes are much more spaced apart. This material is either chemically attacked or subjected to high heat in order to separate the graphite layers. As an example, document WO 2008/060703 (Directa Plus Patent & Technology Limited) describes a method for preparing very thin graphite sheets by exposing intercalated graphite sheets suddenly to temperatures of at least 1300° C. In the method described in the patent application U.S. Patent Application Publication No. 2008/0206124 A1 (inventors: Bor Z. Jang et al.), the material is attacked chemically by halogens, then heated. In another method described in the same document, the material is intercalated by halogens, then subjected, in the liquid phase, to ultrasound. In another method described in patent U.S. Pat. No. 7,081,258 B1 (Nanotek Instruments), the intercalated graphite is subjected to mechanical attrition by very fine grinding.
(E) Ultrasound Synthesis.
A fifth category of method uses ultrasound: different graphite products are dispersed in a liquid and subjected to ultrasound at ambient temperature (see U.S. Patent Application Publication No. 2008/0279756 A1 and U.S. Patent Application Publication No. 2008/0248275 A1 (inventors: Bor Z. Jang et al.).
None of these methods is selective for the synthesis of graphene in the form of monolayers or bilayers or FLG (few-layer graphene), and usually lead to structures in which a plurality, even tens of sheets are stacked. In addition, the graphene obtained by these methods is usually in the form of small sheets, e.g. several nanometers to several tens of nanometers, which are difficult to handle and difficult to deposit on supports for a more in-depth study by the techniques usually used. Moreover, the small size of these sheets makes them difficult to use in potential applications, as it causes in particular joining and assembly edge problems. These sheets have a tendency to fold so as to form complex structures (sometimes called “carbon fractals”, see document U.S. Patent Application Publication No. 2006/0121279 A1); this makes it more difficult to study the fundamental properties of this material, and complicates studies aimed at the potential applications of these materials. The patent application U.S. Patent Application Publication No. 2006/0121279 A1 describes a method for producing carbon nanotubes from graphenes by applying high pressure and high temperature.
Because it is so difficult to obtain, graphene is one of the most expensive materials. Its price may decrease significantly if more effective synthesis methods were available, along the lines of what was observed in the field of carbon nanotubes toward the end of the 1990s in which a major price drop was enabled as synthesis methods were optimized, enabling larger amounts of nanotubes to be obtained with better selectivity, in this case by CCVD (catalytic chemical vapor deposition) growth methods.
In spite of the existence of a number of types of laboratory methods enabling very small amounts of graphenes to be obtained, there is a need to develop new graphene synthesis methods that are more efficient, so as to be capable of studying its physical, chemical and mechanical properties; these studies may make it possible to confront the predictions of different theories with experimental results. In this context, it is also very beneficial to be capable of synthesizing larger graphene sheets, i.e., at least micrometric, so as to better master their deposition on receiving surfaces (substrates) and obtain results that are more easily interpretable and more easily comparable to those obtained for conventional carbon materials.
There is also a need to develop new methods for synthesis of graphene that are simpler and that use inexpensive and abundant starting materials, and to avoid, insofar as possible, the use of chemical reagents or high synthesis temperatures, with the possibility of industrial production, for cases in which graphene has industrial applications; such methods do not currently exist.